Reactively sputtered chrome silicon nitride resistors

ABSTRACT

Radio frequency sputtering of silicon and chromium alloy targets in a nitrogen and argon atmosphere while applying an electrical bias to the substrate produces an electrically resitive thin film on said substrate.

This application is a continuation, of application Ser. No. 505,629,filed June 20, 1983, now abandoned.

The present invention relates to thin films with high electricalresistance and, more particularly, to those which may be formed on asemiconductor integrated circuit.

BACKGROUND OF THE INVENTION

In the construction of semiconductor integrated circuits electricalresistors are often provided as thin films on a surface of asemiconductor body. Due to the limited resistivity of such films and thevery small space available on a typical integrated circuit chip, onlyresistors with comparatively small resistance values may be so provided.Larger resistances are typically provided as discrete elements externalto the integrated circuit. Such external resistors take up a greateramount of space than integrated film resistors, are typically moreexpensive to provide, and are inherently less reliable because of therequirement for mechanical interconnections.

A second problem relates to the stability of the electrical resistanceof a film. Typically such a film is annealed after deposition. If at alater time the film is heated to a temperature approaching the annealingtemperature the resistance value can shift significantly. Film resistorsmay reach such temperatures when operating in a hot environment. Theheat from the ambient conditions combined with heat generated within thesemiconductor body and Joule heating of the resistor can drive theactual temperature of the film to high values. Such shifts in resistancevalues may degrade the performance of a circuit including the resistoror may even prevent the circuit's operation altogether.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of resistivity plotted as a function of nitrogen flowrate for films of the invention formed using a sputtering targetcomprising 27% chromium and 73% silicon,

FIG. 2 is a graph of resistivity plotted as a function of nitrogen flowrate for films of the invention formed using a sputtering targetcomprising 32% chromium and 68% silicon,

FIG. 3 is a graph of the ratio of the change in resistivity caused byannealing to the resistivity prior to annealing plotted as a function ofnitrogen flow rate for films of the invention formed using a sputteringtarget comprising 27% chromium and 73% silicon, and

FIG. 4 is a graph of the ratio of the change in resistivity caused byannealing to the resistivity prior to annealing for films of theinvention formed using a sputtering target comprising 32% chromium and68% silicon.

SUMMARY OF THE INVENTION

The present invention provides a method of producing a high resistivityfilm resistor on a surface of a substrate and the film produced thereby.The film is produced by RF sputtering from a chrome/silicon target in anitrogen/argon atmosphere.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The most common substrate with which the films of the present inventionmay be used is a silicon wafer. Typically the surface of the wafer whichis to receive the film is covered by a thin layer of silicon dioxidewhich provides passivation for the silicon and electrical insulationbetween the resistive films and any semiconductor devices or portionsthereof constructed in the wafer. Semiconductor materials other thansilicon, as could other materials such as ceramics or glass, could beused in the substrate, but as silicon is the most common only it will bediscussed in the explanation and examples below. The silicon dioxidelayer is covered with the resistive film material which is then etchedleaving the resistive film only in those regions where it is desired.

In order to produce the film of the present invention a silicon wafer,having a silicon dioxide layer on the surface on which the film is to beformed, is placed in a standard sputtering system. The sputteringchamber is evacuated to about 2×10⁻⁷ Torr. A nitrogen/argon mixture isthen introduced into the chamber, bringing the pressure to about 5×10⁻³Torr. The partial pressure of the nitrogen represents approximately 5%of the total pressure. A chromium/silicon sputtering target is used. Thetarget is 27 atomic percent chromium, per vendor's assay, the remainderbeing silicon. Such targets are available commercially. A radiofrequency bias of 40 volts at 13.58 MHz is applied to the substrate andthe sputtering is performed with an electrical discharge of 500 watts at13.58 MHz. This power is approximately 10 watts per square inch oftarget area. The substrate being coated is placed on a revolvingplatform to help insure uniform coating in spite of whatevernonuniformities may exist in the distribution of sputtered particlesfrom the sputtering target. During the sputtering nitrogen and argon areintroduced into the system at such rates as will maintain the pressuresat the values indicated above.

The procedure described above will produce a film having a bulkresistivity of about 2500μΩ cm. The resistivity may be varied by varyingthe ratio of chromium to silicon in the sputtering target and hence inthe deposited film or by varying the rate at which nitrogen is suppliedto the sputtering chamber which will, in turn, vary the amount ofnitrogen incorporated in the film.

FIG. 1 is a graph of resistivities plotted as a function of nitrogenflow rate for sputtering targets composed of 27% chromium and 73%silicon. As may be seen from the graph, as the flow rate is increasedfrom zero the resistivity drops from approximately 2100μΩ cm. When thenitrogen flow rate reaches approximately 1.0 standard cubic cm perminute (sccm) the resistivity reaches a minimum value of approximately1200μΩ cm. Further increases in the nitrogen flow rate cause theresistivity to rise. From this it is believed that a change in the basicstructure of the produced film occurs when the nitrogen flow rateexceeds 1.0 sccm as compared with films produced with lower flow rates.

FIG. 2 is similar to FIG. 1 except that resistivities of films producedusing targets composed of 32% chromium and 68% silicon are plotted. Thecurve produced may be seen to be generally similar in shape to the curveof FIG. 1, but shifted downward, i.e. toward lower resistivity values.Therefore, increasing the chromium content of the sputtered film may beseen to decrease resistivity while increasing the nitrogen flow rate,above 1.0 sccm, increases resistivity, as in the films of FIG. 1.

Another important consideration in the manufacture of resistive films isthe stability of such films under thermal stress. As explained above,prior art film resistors tend to suffer large shifts in resistance whenheated to a temperature approaching the initial annealing temperature.Experimentally it has been shown that an indication of the amount ofsuch a shift which can be expected is given by the change in resistancewhich occurs as a result of the initial annealing. FIGS. 3 and 4 plotthe ratio of the change in resistance caused by the annealing to theinitial resistance before the annealing (ΔR/R) as a function of thenitrogen flow rate for films formed from the 27% chromium target and the32% chromium target, respectively. These graphs show a large change inresistance for low nitrogen flow rates. As the flow rate increases theratio of the change in resistance to the initial resistance dropsrapidly until the flow rate reaches approximately 1.0 sccm. At that flowrate the change in resistance levels off at a slightly negative valueand changes very slowly with increased flow rates. These data helpconfirm the belief that nitrogen flow rates over 1.0 sccm produce filmsof different structure than those produced at lower flow rates.

Table 1 below gives values of resistivity (ρ), temperature coefficientof resistance (TCR), and ΔR/R for four specific films manufactured asdescribed above. The values of resistivity are given in μΩ cm and thevalues of TCR are given in parts per million per Celsius degree(ppm/C.°) at a temperature of 100° C. The films characterized in Table 1were annealed at a temperature of 450° C. for thirty minutes in anitrogen atmosphere. The left column of Table 1 indicates chromiumcontent of the targets used to produce the films. In each case theremainder was silicon.

                  TABLE 1                                                         ______________________________________                                        Chromium         Pre-anneal  Post-anneal                                      Content N.sub.2 Flow                                                                           ρ   TCR   ρ TCR   ΔR/R                         ______________________________________                                        45%     4.9 sccm 63,000  -319  66,240                                                                              -244   .051                              27%     3.0 sccm 54,400  -409  58,400                                                                              -278   .074                              45%     4.0 sccm  2,215  -216   2,205                                                                              -153  -.005                              27%     2.0 sccm  2,815  -289   2,685                                                                              -127  -.046                              ______________________________________                                    

As explained above, the small changes in resistivity as a result ofannealing, shown in Table 1, indicate that the films should exhibit verystable resistivities in spite of thermal stress. This is confirmed bythe fact that integrated resistors manufactured according to theinvention exhibited a resistance shift of less than 0.05% after secondand subsequent anneals at the same temperature as the first anneal.Furthermore such resistors exhibited resistance shifts of less than0.01% after 400 hours of storage at a temperature of 150° C. Other testshave shown changes in resistance values of less than 0.01% after filmshave been cycled from between -50° C. and 150° C. ten times. Thisindicates that the films of the invention exhibit good stability undertemperature cycling.

In the process described above a 40 volt bias was applied to thesubstrate on which the resistive film was to be formed. Such a biasvoltage is necessary if the TCR of the film produced is to be close tozero. Films produced with no substrate bias have exhibited TCR's asgreat as -1800 ppm/C.°. As a TCR with a low absolute value is generallydesirable, bias voltages in the range 40 to 60 volts are typicallypreferable. Table 2 indicates the relationship of TCR to bias voltagefor six films produced according to the invention. As in Table 1 thevalues of TCR are for films at 100° C.

                  TABLE 2                                                         ______________________________________                                        Power    Bias         N.sub.2 Flow                                                                           TCR                                            (W/in.sup.2)                                                                           (Volts)      (sccm)   (ppm/C.°)                               ______________________________________                                        10       10           1.9      -218                                           10       40           1.9      -56                                            10       60           1.9      -133                                           10       20           2.9      -324                                           10       40           2.9      -220                                           10       60           2.9      -215                                           ______________________________________                                    

As may be seen from Table 2 the absolute value of the TCR is reducedwhen a film is formed with an electrical bias in the 40 to 60 volt rangeas compared with similar films formed with lower substrate biasvoltages.

The following examples will help to illustrate the invention. Theseexamples are not intended to limit the invention in any way, but serveonly to exemplify specific embodiments of the invention. In each of theexamples the film was formed on a silicon wafer having a thin silicondioxide layer on the surface where the resistive film was formed.

EXAMPLE 1

A film was formed by sputtering from a target composed of 27% chromiumand 73% silicon. An argon flow rate of 37 sccm and a nitrogen flow rateof 3.8 sccm were used to maintain a pressure of 4.2×10⁻³ Torr during thesputtering. A radio frequency discharge of 10 watts/in² relative to thetarget was provided at a frequency of 13.58 MHz. The target was biasedat 950 volts and the substrate on which the film was formed was biasedat zero volts. The film which was formed exhibited a resistivity ofapproximately 82,500μΩ cm and a TCR of approximately -1600 ppm/C.° at100° C. This example illustrates the large absolute value TCR whicharises in films produced with no substrate bias.

EXAMPLE 2

A film was produced by sputtering from a target composed of 27% chromiumand 73% silicon. The sputtering was performed at a pressure of 4.2×10⁻³Torr with an argon flow rate of 39.5 sccm and a nitrogen flow rate of2.0 sccm. A radio frequency discharge of 10 watts/in² relative to thetarget was provided at a frequency of 13.58 MHz. The target was biasedat 950 volts and the substrate at 40 volts. The resulting film had aresistivity of approximately 5150μΩ cm and a TCR of -142 ppm/C.° at 100°C.

EXAMPLE 3

A film was produced by sputtering from a target composed of 27% chromiumand 73% silicon. The sputtering was performed at a pressure of 4.2×10⁻³Torr with an argon flow rate of 39.5 sccm and a nitrogen flow rate of2.60 sccm. A radio frequency discharge of 10 watts/in² relative to thetarget was provided at a frequency of 13.58 MHz. The target was biasedat 950 volts and the substrate at 40 volts. The resulting film exhibiteda resistivity of approximately 25,300μΩ cm and a TCR of approximately-277 ppm/C.° at 100° C.

EXAMPLE 4

A film was produced by sputtering from a target composed of 27% chromiumand 73% silicon. The sputtering was performed at a pressure of 4.2×10⁻³Torr with an argon flow rate of 39.5 sccm and a nitrogen flow rate of2.95 sccm. A radio frequency discharge of 10 watts/in² relative to thetarget was provided at a frequency of 13.58 MHz. The target was biasedat 950 volts and the substrate at 40 volts. The resulting film exhibiteda resistivity of approximately 320,000μΩ cm and a TCR of approximately-410 ppm/C.° at 100° C.

EXAMPLE 5

A film was produced by sputtering from a target composed of 27% chromiumand 73% silicon. The sputtering was performed at a pressure of 4.3×10⁻³Torr with an argon flow rate of 39.7 sccm and a nitrogen flow rate of1.75 sccm. A radio frequency discharge of 10 watts/in² relative to thetarget was provided with a frequency of 13.58 MHz. The target was biasedat 1000 volts and the substrate at 30 volts. The resulting filmexhibited a resistivity of approximately 2575μΩ cm and a TCR ofapproximately -155 ppm/C.° at 100° C.

EXAMPLE 6

A film was produced by sputtering from a target composed of 32% chromiumand 68% silicon. The sputtering was performed at a pressure of 4.2×10⁻³Torr with an argon flow rate of 38.0 sccm and a nitrogen flow rate of3.8 sccm. A radio frequency discharge of 10 watts/in² relative to thetarget was provided with a frequency of 13.58 MHz. The target was biasedat 1000 volts and the substrate at 40 volts. The resulting filmexhibited a resistivity of approximately 24,000μΩ cm and a TCR ofapproximately -261 ppm/C.° at 100° C.

EXAMPLE 7

A film was produced by sputteringg from a target composed of 27%chromium and 73% silicon. The sputtering was performed at a pressure of4.5×10⁻³ Torr with an argon flow rate of 37.0 sccm and a nitrogen flowrate of 3.4 sccm. A radio frequency discharge of 10 watts/in² relativeto the target was provided with a frequency of 13.58 MHz. The target wasbiased at 1050 volts and the substrate at 40 volts. The resulting filmexhibited a resistivity of approximately 30,800μΩ cm and a TCR ofapproximately -224 ppm/C.° at 100° C.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. A method of producing anelectrically resistive thin film on a substrate, said methodcomprising:placing said substrate and a sputtering target comprisingsilicon and chromium in a reaction chamber; evacuating said reactionchamber; providing a sputtering atmosphere comprising nitrogen and argonin said reaction chamber; applying an electrical bias voltage to saidsubstrate; and providing a radio frequency electrical discharge in saidreaction chamber.
 2. The method of claim 1 wherein said electrical biasvoltage is a radio frequency bias voltage.
 3. The method of claim 1wherein said electrical bias voltage is in a range of 40 to 60 volts. 4.The method of claim 1 wherein said nitrogen is provided to said reactionchamber at a rate greater than 1.0 sccm.
 5. The method of claim 2wherein said nitrogen is provided to said reaction chamber at a rategreater than 1.0 sccm.
 6. The method of claim 5 wherein said sputteringtarget comprises chromium in a range of 27 to 45 atomic percent.
 7. Themethod of claim 6 wherein said electrical bias voltage is in a range of40 to 60 volts.
 8. The method of claim 1 wherein said sputteringatmosphere exerts a pressure in a range of 4×10⁻³ to 5×10⁻³ torr whilesaid radio frequency electrical discharge is being provided.
 9. Themethod of claim 8 wherein said electrical bias voltage is in a range of40 to 60 volts.
 10. The method of claim 9 wherein said electrical biasvoltage is a radio frequency bias voltage.
 11. The method of claim 1wherein gas pressure in said reaction chamber is reduced to no greaterthan 2×10⁻⁷ torr during said evacuation step.
 12. The method of claim 11wherein said sputtering atmosphere exerts a pressure in a range of4×10⁻³ to 5×10⁻³ torr while said radio frequency electrical discharge isbeing provided.
 13. The method of claim 12 wherein said electrical biasvoltage is a radio frequency bias voltage.